1. Field of the Invention
The present invention relates to a noncontact communication method for permitting noncontact near-field communication illustratively between an IC (integrated circuit) card and a reader/writer through electromagnetic coupling, and a noncontact communication apparatus for use with that method.
2. Description of the Related Art
An example of the devices based on noncontact communication technology utilizing electromagnetic coupling is FeliCa (registered trademark). The NFC (Near Field Communication) Standard is a representative standard for near-field communications.
FIG. 30 is a schematic view explanatory of a noncontact communication system using the technology of FeliCa (registered trademark). This noncontact communication system is composed of a reader/writer 10 and a transponder 20.
In a well-known example shown in FIG. 31, the transponder 20 is provided in the form of a noncontact IC card, and the reader/writer 10 is installed illustratively in the ticket gates of railway stations or in automatic vending machines. A user 30 brings the transponder 20, which is the noncontact IC card held in his or her hand, close to the reader/writer 10 for personal identification and billing.
The reader/writer 10 is made up of an antenna resonance circuit 11 and a transmission/reception processing block 12. The antenna resonance circuit 11 is formed by a coil 11L for parallel resonance, a capacitor 11C, and a resistor 11R.
The transmission/reception processing block 12 includes a detection circuit and a demodulation circuit for detecting and demodulating an incoming signal, a control block of the reader/writer 10, and a modulation circuit for modulating an outgoing signal. The transponder 20 contains an antenna resonance circuit 21, a load change modulation circuit 22, and a transmission/reception processing block 23.
The antenna resonance circuit 21 is constituted by a coil 21L for parallel resonance, a capacitor 21C, and a resistor 21R. The load change modulation circuit 22 is composed of a resistor 22R and a semiconductor switch 22SW. The transmission/reception processing block 23 includes a detection circuit and a demodulation circuit for detecting and demodulating the incoming signal, a control block, and a modulation circuit for modulating the outgoing signal.
The semiconductor switch 22SW is controlled by a signal coming from the transmission/reception processing block 23. Upon receipt of the signal, the semiconductor switch 22SW is turned off. At transmission time, the semiconductor switch 22SW is turned on and off by the signal modulated to be transmitted.
The noncontact IC card constituting the above-described transponder 20 is a non-powered card. As such, the card obtains its own DC drive power by rectifying an induced current provided by the reader/writer 10 through electromagnetic induction.
Where the near-field communication system above is in use, the electromagnetic coupling between the antenna resonance circuit 21 of the transponder 20 and the antenna resonance circuit 11 of the reader/writer 10 permits communication between the transponder 20 and the reader/writer 10. In this case, the two parties involved in near-field communication typically utilize a carrier frequency of 13.56 MHz. Communication between the reader/writer 10 and the transponder 20 takes place at a transmission/reception distance D of 0 cm (contact state) through 10-plus cm.
The antenna resonance circuits 11 and 21 for the reader/writer 10 and transponder 20 are both designed to have a steep resonance frequency characteristic near the carrier frequency. The steep resonance frequency characteristic is provided so that the non-powered transponder 20 may acquire sufficient DC drive power and that the reader/writer 10 and transponder 20 may both obtain sufficient modulation signal intensity.
Illustratively, FIG. 32 shows a typical resonance frequency characteristic of the Edy (registered trademark) card used extensively in connection with the FeliCa (registered trademark) communication system. The resonance frequency characteristic shown in FIG. 32 represents measurements taken when the transmission/reception distance D between the reader/writer 10 and the transponder 20 is about 1 cm. The vertical axis of the graph in FIG. 32 denotes normalized parameters obtained by measuring instruments. The Q (quality) factor indicative of the steepness of resonance in the characteristic curve of FIG. 32 is approximately 49. The larger the Q-factor, the steeper the frequency characteristic of resonance.
The noncontact IC card for use with the above-outlined near-field communication system is disclosed illustratively in Japanese Patent Laid-Open No. Hei 10-187916 (called the Patent Document 1 hereunder) and Japanese Patent Laid-Open No. 2005-11009 (called the Patent Document 2 hereunder).